r/'.RJSSODACTYLA 369 



in size and structure. Crown of the last lower molar commonly 

 bilobed, and if a third lobe is present in this tooth it is wanting in 

 the last lower milk-molar. Dorso-lumbar vertebras never fewer than 

 twenty-two, usually twenty-three in the existing species. Nasal 

 bones expanded posteriorly. An aUsphenoid canal. Femur with 

 a third trochanter. 1 The middle or third digit on both fore and 

 hind feet larger than any of the others, and symmetrical in itself, 

 the free border of the ungual phalanx being evenly rounded (see 

 Fig. 1 5 1 ). This may be the only functional toe, or the second and 

 fourth may be subequally developed on each side of it. In the 

 Tapirs and many extinct forms, the fifth toe also remains on the 

 fore limb, but its presence does not interfere with the symmetrical 

 arrangement of the remainder of the foot around the median line 

 of the third or middle digit. Traces of a hallux have only been 

 found in some extremely ancient and primitive forms. The 

 astragalus has a pulleydike surface above for articulation with the 

 tibia, but its distal surface is flattened and unites to a much greater 

 extent with the navicular than with the cuboid, w T hich bone is 

 of comparatively less importance than in the Artiodactyla. The 

 calcaneum does not articulate with the lower or distal extremity of 

 the fibula. The stomach is always simple, the caecum is large and 

 capacious, the placenta diffused, and the mammae are inguinal. 

 The gall-bladder is invariably absent. 



As regards the dentition, the whole of the premolar series 

 may be preceded by milk-teeth ; and it has been demonstrated in 

 Rhinoceros that Avhen there is no displacement of the first cheek- 

 tooth that tooth is a persistent milk-molar ; the same condition 

 apparently holding good in Palceotherium. This feature indicates 

 considerable dental specialisation, the milk-molars, according to the 

 theory generally accepted by the leading English zoologists, being 

 the acquired, and the premolars the original series. Another 

 peculiar feature of the dentition of the Perissodactyla, very rarely 

 met with among the Artiodactyla, is that the premolars tend to 

 resemble the true molars ; this feature occurring in all the existing 

 genera, although not found in the earlier generalised types. The 

 cheek-teeth of all the members of the suborder are primarily con- 

 structed on some modification of what is known as the lophodont 

 plan. Thus the upper molars (Fig. 155, p. 375) have an outer antero- 

 posterior wall from which proceed two transverse ridges, formed by 

 the coalescence of the primitive inner and outer columns, towards 

 the inner aspect of the crown ; while in the lower molars there 

 may be either two simple transverse ridges, or these ridges may be 

 curved into crescents, coming into contact with one another at their 

 extremities. Those forms having brachydont teeth show this plan 

 of structure in its simplest modification ; but in cases, as in the 



1 Wanting in the ulicrrant < 'I/n/ir,,fh< ri",n. 



24 



